Phasic and tonic reflexes evoked in human antagonistic wrist muscles by tendon vibration

The electromyographic reflex responses of the voluntarily contracting wrist flexor and extensor muscles to periods of vibration-evoked enhanced, Ia-dominated afferent discharge from flexor carpi radialis (FCR) were studied in normal human subjects. Three main response phases were characterised, namely, (i) phasic 'on' responses elicited at the commencement of stimulation, (ii) tonic response levels occurring during prolonged stimulation and (iii) phasic 'off' responses elicited at the termination of stimulation. The phasic 'on' reflex responses of FCR and extensor carpi radialis (ECR) comprised, respectively, a peak of autogenetic excitation of group mean latency 18.8 ms and a trough of reciprocal inhibition of group mean latency 38.0 ms. Prolonged (2 s) trains of FCR (agonist) vibration evoked a phase of tonic reflex excitation in FCR whose mean level was significantly increased, by 20%, above pre-stimulus activity and which did not change over the 0.5-2.0 s vibration period. Progressive reduction of the duration (from 2000 ms to 100 ms) of vibration trains demonstrated that phasic disfacilitatory 'off' troughs regularly occurred, with a consistent latency (mean 24.2 ms), on withdrawal of each period of enhanced Ia-input. This indicates that the responsible excitatory reflex mechanism was operational for the entire duration of each of the vibration periods tested. The extra latency (on average 5.4 ms) of phasic 'off' relative to 'on' responses may be attributed to factors (e.g. 5-10 ms duration of unitary muscle action potentials and afterdischarge in reflex pathways) which inevitably delay the appearance of overt disfacilitatory reductions in EMG rather than the involvement of different reflex pathways. Thus, short-latency, possibly monosynaptic, reflex excitation contributed throughout the entire tonic excitatory response. Sustained FCR (antagonist) vibration produced a significant tonic reciprocal inhibitory reflex depression, by 7% pre-stimulus EMG, of ECR activity which remained steady during the 0.5-2.0 s vibration period. The absence of well-defined phasic disinhibitory 'off' responses in ECR suggests that the contribution of oligosynaptic reflex inhibitory mechanisms to the tonic suppression of activity occurring during continuing vibration is relatively small.